Llanelli, who had earlier lost 22-13 in Cardiff, stopped the Blue and Blacks from completing the double by edging them out 13-11 at Stradey Park in the Under-21's Premier Division.
Nick Macleod, who recently made his senior debut against Cambridge University, kicked the only points of the first-half with a penalty that gave Cardiff the lead at 3-0.
Llanelli came back into the match when acting captain Aled Gravell scored a try and he was soon followed over the line by prop forward Phil John with fly half Phil Fisher kicking a penalty goal.
At 13-3, Cardiff retaliated with a try by wing Geraint Williams and a second penalty by Macleod, but the latter's conversion attempt that would have provided the equalising points went wide.
Llanelli turned out their Academy 17-year-olds against Bath at Stradey Park and pulled off an excellent 18-0 win.
Wing Dafydd Evans ran the first try in from 50 metres and full back Ricky Williams placed a 30-metre penalty to lead 8-0 at the break.
In the second spell, centre Aled Thomas raced in for the second Scarlet try and a smart pass by fly half Roger Davies put Williams away for the final score.
Earlier, Cardiff's under-21's had scored a resounding 41-15 win at Swansea, to avenge their 31-29 loss at home in the first meeting. It was one of the best Cardiff performances that included tries by full back Owain Ashman, scrum half Scott Heddington, lock Luke Tait, wings Elgan Jones and Geraint Williams and centre Liam Roberts.
Nick Macleod placed four conversions and a penalty, but Swansea had led 15-13 at half-time and then lost lock Bill Brereton with a broken ankle. Prop Steve Tremlett and Brereton scored their tries with fly half Gareth James adding a penalty and conversion.
Having taken part in seven World Cups as a player and coach, Liza Burgess hopes to go out in style this afternoon when Wales take on Australia in the Play-off for seventh and eighth place in the Women's Rugby World Cup in Paris.
WRU Head of Rugby Josh Lewsey met with volunteers across the game earlier this week, championing their role in the sport. He was joined by Wales captain Sam Warburton for part of the day as volunteers took part in a 'Try Out' event for the 2015 Rugby World Cup
Wales Women may not have got the results they wanted recently on the field, but off it they have made massive strides in strength and conditioning - and that is in no small part due to S&C coach Sam Dodge.
Two weeks into the new pitch project at Millennium Stadium, Groundman Craig Campbell takes staff on a unique behind-the-scene tour of the works so far and National Squad Assistant Strength and Conditioning Coach John Ashby and Tour Guide Peter Owen describe what the revamp means for them.
Try-scorer Jenny Davies and fellow prop Megan York reveal to WRU TV's Graeme Gillespie their thoughts after Wales broke their drought in the Women's Rugby World Cup with a 35-3 win over South Africa in Paris.
Wales Women are expecting a tough battle against South Africa on Saturday as both teams seek their first pool win of the Rugby Women's World Cup. WRU TV's Graeme Gillespie talks to head coach Rhys Edwards and lock Jenny Hawkins.
Wales' Sam Warburton took a group of employees from Admiral on a behind the scenes tour of the Millennium Stadium and WRU TV tagged along to bring you a snippet of what it's like for the players on match day
WRU TV goes behind the scenes in Paris with adopted reporter Jenny Davies grilling Wales Women's captain Rachel Taylor and uncovers which players she would have on a desert island, among other important things as well.
Prop Caryl Thomas was part of a dominant Welsh pack but still found herself on the receiving end of a 25-3 defeat to Australia in the Women's Rugby World Cup. WRU TV's Graeme Gillespie got her post match reaction.